Water challenges
Entre estos, se incluyen la sobreexplotación de agua subterránea, el hundimiento de tierra, el riesgo de grandes inundaciones, el impacto de la creciente urbanización, la baja calidad del agua y el suministro intermitente, el uso ineficiente del agua, un índice bajo de tratamiento de aguas residuales, problemas de sanidad sobre la reutilización de aguas residuales para riego y la recuperación limitada de costos para el agua. La cobertura de la infraestructura en términos de acceso a una conexión de conductos de agua o al saneamiento, la cual se utiliza para supervisar los Objetivos de Desarrollo del Milenio para el abastecimiento de agua y el saneamiento, es casi universal en la Zona Metropolitana del Valle de México y como tal no constituye un reto.
Overexploitation of groundwater
The exponential growth of the population of the Mexico City Metropolitan Area has depleted its groundwater resources. The recharge of the aquifer is about 31.6 m/s which, when compared to the extraction of 59.5 m/s, results in an excess consumption of 28 m/s.[10] In 1983, systematic monitoring of the aquifer's water levels began. Since that time, the average annual decline in groundwater levels varies from 0.1 to 1.5 meters per year in different areas. Taking into account the current rate of depletion, the estimated storage volume has been calculated to be between 200 and 350 times the annual extraction. However, with a simplistic approach to the water balance, other realities are being ignored. For example, the aquifer is vulnerable to geologically induced water quality problems with increasing aquifer consolidation and depth and fragmentation of clay layers. Furthermore, it is possible that the actual available volume of the main aquifer was lower than estimated due to the probable reduction in porosity with increasing depth. There are also practical and economic limitations to the pumping depth.[11].
What follows is the water balance of the Metropolitan Zone of the Valley of Mexico:
land subsidence
The city rests on the highly saturated clay of ancient Lake Texcoco. This soft foundation is collapsing due to over-extraction of groundwater. Land subsidence in Mexico City caused by overexploitation of groundwater over the last hundred years has been up to 9 meters and, as a consequence, damage has occurred to buildings, streets, sidewalks, sewers, stormwater drains and other infrastructure.
Flood
Flooding is common in Mexico City and swamps roads and sidewalks. In low-lying neighborhoods like Iztapalapa, residents are so accustomed to seeing a fetid sea of sewage rising in the streets that they have built small dams in front of their houses.[5] Floods are caused by the joint subsidence and increased impermeability of the soil due to urbanization. According to the modeling, if the Central Emitter failed during the rainy season, a major flood would occur that would flood the historic center, the Mexico City International Airport, and the eastern districts of Mexico City.[12].
Increase in urbanization
Urban growth reduces soil permeability in groundwater recharge zones and increases the risk of flooding. It is estimated that the metropolitan area grows annually from 200 to 300 hectares in the recharge areas. For every square meter that is built, an average of 170 liters of recharge is lost per year. In this way, for every hectare that is built, water is lost for 500 families and the groundwater pressure additionally increases.[13] Between 1989 and 1994, the Federal District declared six environmental protection zones, mainly in forests and hillsides, with a total area of almost 5,000 hectares in order to protect them from urbanization.[14].
Water quality and intermittent supply
Poor water quality is a concern at the source and point of use. Initially, it was believed that Mexico City's groundwater was protected from contamination by a thick impermeable layer. However, this layer has been fragmented due to land subsidence. Likewise, the natural upward flow of water in the Artesian Well has now been reversed due to groundwater depletion. In this way, it is believed that Mexico City's groundwater will become increasingly vulnerable to contaminants from landfills and industrial sites that leach into the aquifer. In addition, the deep drainage system penetrates the clay (aquitard) in some places of the main aquifer. During periods of heavy rain, wastewater leaks out of the deep tunnels into the surrounding subsoil and causes contamination of the aquifer.[15].
Distribution system leaks are one of the main sources of concern for drinking water quality. When wastewater enters the ground from leaking sewers or other sources, when the pressure is low the leaking pipes become infiltrated with contaminated water. According to the Federal District Water Quality Laboratory, neighborhoods that experience more frequent service interruptions have worse quality water than neighborhoods with constant supply. The percentage of household tap water samples in compliance with the residual chlorine standard (0.2 milligrams/liter) was 87 to 100 percent of the tests carried out in 1993 and was notably low in the southeastern delegations (Iztapalapa, Tláhuac and Xochimilco). Household water storage tanks or tinacos are common on almost all house roofs and usually store water when the system's water pressure is inadequate. In many cases, the tanks are open and not cleaned regularly, which causes residual chlorine to dissipate and encourage the growth of microorganisms. The standard levels of chlorine (0.2 milligrams/liter) maintained in the distribution system upon reaching the customer's tap are not sufficient to eliminate microorganisms that may have penetrated the pipes.[15].
The water supply in many parts of the Metropolitan Area of the Valley of Mexico is intermittent and the pressure is usually insufficient. Thus, users have to supplement their water supply with water purchased from tanker trucks or pipes. Occasionally, the water supply is even interrupted for several days, as happened in January 2009 when the Cutzamala system's water supply had to be reduced, cutting off water to 5.5 million people for three days.[16] This incident was followed by a second supply interruption in March 2009 and, more recently, a third interruption in April 2009. This third interruption was made in response to the alarmingly low levels of the city's water reserves. and carrying out repairs in the Cutzamala system. This lasted 36 hours and left more than five million city residents without water services. In response, the Mexico City government has had to implement an emergency supply response program, whereby water is provided in tanker trucks and bottles to residents of affected communities.[4].
Limited wastewater treatment and concerns about reuse for irrigation
As mentioned previously, only 15% of the wastewater in the Metropolitan Area of the Valley of Mexico currently receives treatment. Stormwater, raw municipal wastewater and partially treated industrial wastewater are mixed together and reused for large-scale irrigation.
There are concerns about the health and environmental impact of reusing raw wastewater from Mexico City for irrigation. Crops grown with wastewater are limited to crops that are not eaten raw, but these limitations are difficult to enforce and farmers also grow vegetables using wastewater. According to a study by the International Water Management Institute (IWMI), these risks must be carefully considered, but the importance of this practice for the livelihoods of countless smallholders must also be taken into account.[17] Studies on soils irrigated with untreated wastewater for 50 years reveal an accumulation of heavy metals in the soil, but also that they accumulate to a lesser extent in plants.[18] According to another study, bacterial contamination has been found in the water of the canals used for bathing and in the groundwater used to supply drinking water from irrigated areas where wastewater has been reused, which has led to a high incidence of diarrhea and skin irritations.[19].
Inefficiency in urban water use
The Federal District has a non-profitable water level of 40%, close to the national average, which implies that only 60% of the water pumped into the system is actually billed. A large percentage of unprofitable water is not due to leaks, but to illegal connections. Additionally, SACM, the district's water department, has by far the lowest collection efficiency among the top 25 municipalities, only receiving payment for 40% of all its bills. Therefore, only 24% (60% billed, 40% bills paid) of the water pumped into the system is paid for. The level of non-profitable water in the State of Mexico is lower, so that the average level of the metropolitan area is 36%.
Figures regarding per capita water use are difficult to compare over time, as sources typically do not indicate whether water losses are included in the figures or not. The National Institute of Statistics and Geography established the Federal District's water use at 223 liters/day in 1999 (probably after losses), including 164 liters for residential use and 59 liters for industrial and commercial uses. This is only one-third of the average US water use, which is 603 liters/person/day. However, it is still a third higher than water use in France, which is only 165 liters/person/day.[20] Another source establishes the average per capita water use in 1994 between 364 and 230 liters per day for the Federal District and the State of Mexico respectively (probably before losses). Authorities attribute the increase in per capita use in the Federal District to the fact that it is more developed and has more commercial and industrial activity than the State of Mexico.[1] However, lower rates and metering in the Federal District can also influence the greater use of water.
Limited cost recovery
There is an enormous gap between the cost of water supply, a quarter of what is imported through inter-basin transfers from the Lerma and Balsas basins, and what is recovered from users. Among the reasons for the low cost recovery are illegal water connections, low rates and the low level of bill collection, specifically in the Federal District.
Cost recovery in the State of Mexico is much higher than in the Federal District. For example, the city of Toluca in the State of Mexico charges residential users 9.5 pesos (US$0.72)/m for a consumption of 30 m³, including a sanitation surcharge, the sixth highest rate among the country's top 25 municipalities.[21] This is still insufficient relative to the cost of bulk water from the Cutzamala system, at almost 10 pesos (US$0.78)/m, however. take into account the cost of water distribution and sanitation.
In 2004, the Federal District charged residential users 3.3 pesos (US$0.26)/m for the same consumption without any sanitation surcharge, the fourth lowest rate among the same municipalities.[21] The rest is effectively subsidized through the municipal and federal governments. In August 2007, a conflict arose between CONAGUA and the Federal District when CONAGUA increased the rate for water supplied through the Cutzamala system and the local government rejected it.